Calculate the excess chemical potential, i.e. G = G_Real G_Ideal molar, in the isothermal expansion of 1 mol of real gas, at 283K, from 120 bar to 20 bar if the gas behaves as represented by the van der Waals equation in the form of virial expansion with only one term in P , Z = 1 + [1/(RT)].[b - a/(RT)].P, with values of a = 1.390 (atm.L .mol) and b = 0.039 (L.mol) . Express the result in (J/mol).